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Controlled thinning of wide bandgap semiconductors by chemo-mechanical polishing (CMP) and/or reactive ion etching (RIE) has been one of the versatile methods for various optoelectronic applications. The influences of CMP and subsequent wet chemical etching, as well as independent RIE, on the room-temperature photoluminescence lifetime for the near-band-edge emission [ τ PL NBE (RT) ] of O-polarity c-plane ZnO single crystals were examined by using time-resolved photoluminescence measurements. τ PL NBE (RT) decreased from a nanosecond range to a few picoseconds (ps) by a conventional CMP, indicating a generation of high-concentration midgap recombination centers, such as nonradiative recombination centers and deep radiative recombination centers. τ PL NBE (RT) was progressively regained up to 600 ps by a subsequent etching using HCl aqueous solution. However, the recovery saturated at the etching depth of about 200 nm and τ PL NBE (RT) was not restored even after etching by 350 nm. The results indicate the introduction of certain structural deformations during the CMP. Because x-ray diffraction measurement revealed the presence of incoherent surface domains right after the CMP and the HCl etching gave rise to inhomogeneously etched canyons, nonradiative recombination centers, such as dislocations and vacancy clusters, are likely generated by mechanical shear stresses. τ PL NBE (RT) also decreased by the RIE. However, the degradation was less significant than the case for the CMP, because RIE scarcely gives mechanical stresses. Interestingly, τ PL NBE (RT) for the samples etched under higher plasma power was longer than the lower power cases. From the results of x-ray photoelectron spectroscopy measurements, unintentionally deposited oxide films containing Si are proposed to act as an attenuating layer for the introduction of nonradiative recombination centers. [ABSTRACT FROM AUTHOR]

Lattice-matched Al1−xInxN / GaN heterostructures with InN mole fraction (x) of 0.18 have attracted considerable interest for use in GaN-based optoelectronic devices. Because the light emission efficiency (η emission ) of Al1−xInxN alloys is far less than that of InxGa1−xN, understanding its causes is essential. For this purpose, room-temperature photoluminescence lifetime (τ PL RT ), which almost represents the nonradiative recombination lifetime that limits the internal quantum efficiency in low η emission semiconductors, of c-plane Al1−xInxN epilayers nearly and modestly lattice-matched to GaN (0.11 ≤ x ≤ 0.21) was examined. For the epilayers grown on low threading dislocation density (TDD) GaN substrates (≪ 10 7 c m − 2 ), τ PL RT principally decreased with increasing x, indicating a progressive increase in the concentration of nonradiative recombination centers (NRCs), N NRC . One of the probable causes is the growth temperature (T g ) reduction that is indispensable to incorporate more In, because in insufficient T g regime higher T g is preferred for enhancing the surface migration of adatoms to decrease the concentrations of vacancies that compose NRCs. The Al1−xInxN epilayers of the same x but grown on high TDD (> --> 10 8 c m − 2 ) GaN-on-sapphire templates exhibited shorter τ PL RT . Because the diffusion length of minority carriers was nearly zero in the Al1−xInxN epilayers, the shorter τ PL RT indicates higher bulk N NRC in high TDD epilayers. The Al1−xInxN epilayers of considerably rough surface morphologies exhibited spatially inhomogeneous τ PL RT , implying that excited carriers recombined everywhere at InN-rich to InN-poor portions, where N NRC were likely lower to higher, respectively, than the average due to the deviations in the surface stoichiometry at various non-c-plane surfaces at a given T g . [ABSTRACT FROM AUTHOR]

Prospective equivalent internal quantum efficiency (eta(int)) of approximately 34% at 300 K was demonstrated for the blue emission peak of nonpolar m-plane (1 (1) over bar 00) InxGa1-xN/GaN multiple quantum well light emitting diodes (LEDs) fabricated on freestanding m-plane GaN substrates. Although the eta(int) value is yet lower than that of conventional c-plane blue LEDs (> 70%), the results encourage one to realize high performance green, amber, and red LEDs by reducing the concentration of nonradiative defects, according to the absence of the quantum-confined Stark effects due to the polarization fields parallel to the quantum well normal. The electric field component of the blue surface emission was polarized perpendicular to the c axis with the in-plane polarization ratio of 0.58 at 300 K. (c) 2006 American Institute of Physics.

Recombination dynamics of the 268 nm photoluminescence (PL) peak in a quaternary Al0.53In0.11Ga0.36N/Al0.58In0.02Ga0.40N multiple quantum well (MQW) grown on relaxed AlGaN templates were studied. Although the polarization field in the compressively strained Al0.53In0.11Ga0.36N wells was as high as 1.6 MV/cm, the value of integrated PL intensity at 300 K divided by that at 8 K (eta(int)) was as high as 1.2%. The value was similar to that obtained for the 285 nm PL peak in an Al0.30Ga0.70N/Al0.70Ga0.30N MQW (1.3%), though the A1N molar fraction in the wells was higher by a factor of 1.7. According to these results and the fact that time-resolved PL signal exhibited a stretched exponential decay shape, the improved eta(int) of the AlInGaN wells was attributed to a beneficial effect of the exciton localization as is the case with InGaN alloys; doping or alloying with InN was confirmed to work also on AlGaN in improving eta(int) to realize deep UV optoelectronic devices.

Cathodoluminescence technique combined with transmission electron microscopy (TEM-CL) has been used to characterize optical properties of dislocations in GaN epilayers. The dislocations act as nonradiative centers with different recombination rates. TEM-CL observation showed that even for the same Burgers vector of a, the dislocations show different electrical activity depending on the direction of dislocation line, i.e., the edge-type dislocation parallel to the c plane is very active, while the screw-type one is less active. The simulation of the CL images gives us the information of parameters such as carrier lifetime and diffusion length. (C) 2003 American Institute of Physics.

The nonradiative lifetime (τNR) of the near-band-edge emission in various quality GaN samples is compared with the results of positron annihilation measurement, in order to identify the origin and to determine the capture-cross-section of the major intrinsic nonradiative recombination centers (NRCs). The room-temperature τNR of various n-type GaN samples increased with decreasing the concentration of divacancies composed of a Ga vacancy (VGa) and a N vacancy (VN), namely, VGaVN. The τNR value also increased with increasing the diffusion length of positrons, which is almost proportional to the inverse third root of the gross concentration of all point defects. The results indicate that major intrinsic NRC in n-type GaN is VGaVN. From the relationship between its concentration and τNR, its hole capture-cross-section is estimated to be about 7 × 10−14 cm2. Different from the case of 4H-SiC, the major NRCs in p-type and n-type GaN are different: the major NRCs in Mg-doped p-type GaN epilayers are assigned to multiple vacancies containing a VGa and two (or three) VNs, namely, VGa(VN)n (n =2 or 3). The ion-implanted Mg-doped GaN films are found to contain larger size vacancy complexes such as (VGa)3(VN)3. In analogy with GaN, major NRCs in Al0.6Ga0.4N alloys are assigned to vacancy complexes containing an Al vacancy or a VGa. [ABSTRACT FROM AUTHOR]

Research was conducted to examine the structural and vibrational features of device quality pure GaN substrate grown using a lateral epitaxial ovvergrowth method by x-ray diffraction, Brillouin, Raman and infrared spectroscopy. Lattice constants were observed to be close to bulk GaN and relaxed epilayers on sapphire. Results indicate extremely good structural quality of the GaN substrate with structural coherence length of as long as 1.9 micron along the c axis.

High optical-quality polycrystalline and epitaxial CuInSe2 thin films were grown by a simultaneous-feeding or alternate-feeding physical vapor deposition process. The presence of the liquid-phase Cu(sub 2-x)Se during growth was verified in terms of meltback of the substrate. The grown films had predominant near-band-edge photoluminescence peaks from 2 K up to room temperature. A clear free exciton absorption peak was detected and its full width at half maximum decreased with decreasing repetition periods of the metal feeding cycle.

It has been established that the band gap energy of both CuInSe2 and CuGaSe2 is essentially independent of the molar ratio to Cu to group-III atom (Cu/III) for near-stoichiometric and Cu-rich samples. Plasma screening of Coulomb interactions prompts the disappearance of the free excitation absorption in In-rich CuInSe2 thin films. The fundamental band gap energies of both strain-free CuInSe2 and CuGaSe2 are almost constant for the samples having Cu/II greater than 0.88.

Study of the low-temperature photoluminescence (PL) spectra of CuAlSe2 epilayers reveals that the PL peaks are associated with a free exciton and a bound excitons. The CuAlSe2 epilayers are grown on GaAs(001) substrates by using low-pressure metal organic chemical-vapor deposition. The PL properties are investigated with reference to the metal organic precursors required for the growth of the epilayers.

Low pressure metalorganic chemical-vapor deposition helps develop CuGaSe2 chalcopyrite compounds on GaP and GaAs substrates whose structural and optical characteristics and heteroepitaxy are analyzed. The near-band-gap domain in the epilayer contains a strong photoluminescence (PL) peak. An intense peak at 1.71 eV is observed in low temperature PL spectra. A change from free to acceptor at 1.66 eV results in a weak PL peak.

The characteristics of Si-doped GaAs epilayers grown by low-pressure metalorganic chemical vapor deposition (MOCVD) using tertiarybutylarsine were investigated using a monoenergetic positron beam. The results showed that heavy Si-doping levels caused a significant increase in the concentration of Ga vacancies. The Ga vacancy concentration in samples grown by MOCVD was lower than that in samples grown by molecular beam epitaxy.

A record-long room-temperature photoluminescence (PL) lifetime ( τ PL RT ) of approximately 70 ps was obtained for the sub-bandgap 3.7 eV emission band of a 300-nm-thick c-plane Al0.83In0.17N epilayer for the use in cladding layers of an edge laser structure, which were grown by metalorganic vapor phase epitaxy on a low threading dislocation density nearly lattice-matched GaN substrate. The recorded τ PL RT value was twice as long as previously reported ones, indicating half concentration of nonradiative recombination centers. Room-temperature spatially resolved cathodoluminescence intensity images for the 3.7 eV band revealed nearly zero carrier diffusion length, which is consistent with the fact that τ PL RT of 70 ps is 1/35 of the near-band-edge emission of the GaN substrate (2.4 ns). As the PL decay curves for the 3.7 eV band were sufficiently fitted by the stretched exponential function, the emission likely originates from extended states such as impurities, point defects, and their complexes, as well as localized states of uneven potential profile. [ABSTRACT FROM AUTHOR]

Structural and optical qualities of half-a-lm-thick m-plane Al1-xInxN epilayers grown by metalorganic vapor phase epitaxy were remarkably improved via coherent growth on a low defect density m-plane freestanding GaN substrate prepared by hydride vapor phase epitaxy. All the epilayers unexceptionally suffer from uniaxial or biaxial anisotropic in-plane stress. However, full-width at half-maximum values of the x-ray ω-rocking curves were nearly unchanged as the underlayer values being 80∼150 arc sec for (1010) and (101 2) diffractions with both (0001) and (1120) azimuths, as long as pseudomorphic structure was maintained. Such Al1-xInxN epilayers commonly exhibited a broad but predominant luminescence peak in ultraviolet (x⩽0.14) to green (x=0.30) wavelengths. Its equivalent value of the internal quantum efficiency at room temperature was as high as 67% for x=0.14 and 44% for x=0.30. Because its high-energy cutoff commonly converged with the bandgap energy, the emission peak is assigned to originate from the extended near-band-edge states with strong carrier localization. [ABSTRACT FROM AUTHOR]

Luminescence dynamics for the near-band-edge (NBE) emission peak at around 250 nm of c-plane Si-doped Al0.6Ga0.4N films grown on AlN templates by low-pressure metalorganic vapor phase epitaxy were studied using deep ultraviolet time-resolved photoluminescence and time-resolved cathodoluminescence spectroscopies. For the films with the Si-doping concentration, [Si], lower than 1.9 × 1017 cm-3, the doping lessened the concentration of cation vacancies, [VIII], through the surfactant effect or the aid of the reactant doping in a form of H3SiNH2. However, the room-temperature nonradiative lifetime, and, consequently, the equivalent value of internal quantum efficiency in the weak excitation regime steeply decreased when [Si] exceeded 1018 cm-3. Simultaneously, the intensity ratio of the deep-state emission band to the NBE emission abruptly increased. Because the increase in [Si] essentially gives rise to the increase in [VIII] (for [Si]>1.9×1017 cm-3) and the overcompensation of Si is eventually observed for the film with [Si] = 4.0 × 1018 cm-3, the formation of acceptor-type native-defect complexes containing Si such as VIII-SiIII is suggested. [ABSTRACT FROM AUTHOR]

Research was conducted to study band-gap separation in InGaN epilayers prepared through metalorganic chemical vapor deposition. Static photoluminescence was excited by a cw He-Cd laser while the average InGAN molar fraction was determined using x-ray diffraction measurements. Results showed that each separated region of the band gap had homogeneous net potential fluctuation and unchanged homogeneous net potential fluctuations.

Vacancy-type defects in Mg-doped GaN grown by metalorganic vapor phase epitaxy were probed using a monoenergetic positron beam. For a sample fabricated with a high H2-flow rate, before post-growth annealing the major defect species detected by positrons was identified as vacancy-clusters. Evidence suggested that other donor-type defects such as nitrogen vacancies also existed. The defects increased the Fermi level position, and enhanced the diffusion of positrons toward the surface. The annihilation of positrons at the top surface was suppressed by Mg-doping. This was attributed to the introduction of a subsurface layer (<6 nm) with a low defect concentration, where the Fermi level position was considered to decrease due to partial activation of Mg. For samples after annealing, the trapping of positrons by residual vacancy-type defects was observed, and the sample crystal quality was found to depend on that before annealing. [ABSTRACT FROM AUTHOR]

The helicon-wave-excited-plasma sputtering (HWPS) method was exemplified to be one of the versatile epitaxial growth techniques for the fabrication of low dislocation density semiconductor epilayers and heterostructures exhibiting atomically smooth surface morphology. For a case study, ZnO homoepitaxy and MgxZn1-xO (x=0.08,0.19) heteroepitaxy on a Zn-polar ZnO substrate were carried out. According to the surface damage-free property, high temperature growth with appropriate stoichiometry control enabled the growth of ZnO homoepitaxial layers exhibiting a smooth surface morphology with 0.26 nm high monolayer atomic steps. Their tilt and twist mosaics reflecting the threading dislocation densities having screw and edge components were comparable to those of the substrate, being under the resolution limit (18 arcsec). The surface morphology and crystal mosaicity of pseudomorphic MgxZn1-xO (x≤0.19) epilayers were quite similar to those of the ZnO underlayer. The luminescence spectra of the ZnO and MgxZn1-xO epilayers at 293 K exhibited a predominant near-band-edge emission and negligible broad emission bands due to deep levels. The results indicate that the growth mode of the HWPS method resembles that of molecular beam epitaxy methods. [ABSTRACT FROM AUTHOR]

Polarization characteristics of the near-band-edge optical transitions in m-plane AlxGa1-xN epilayers suffering from anisotropic stresses were quantified. The epilayers were grown by both ammonia-source molecular beam epitaxy and metalorganic vapor phase epitaxy methods on an m-plane freestanding GaN substrate. The light polarization direction altered from E⊥c to E∥c at the AlN molar fraction, x, between 0.25 and 0.32, where E is the electric field component of the light and ⊥ and ∥ represent perpendicular and parallel, respectively. To give a quantitative explanation for the result, energies and oscillator strengths of the exciton transitions involving three separate valence bands were calculated as functions of strains using the Bir–Pikus Hamiltonian. The calculation predicted that the lowest energy transition (E1) is polarized to the m-axis normal to the surface (X3) for 01 emission is principally undetectable from the surface normal for any in-plane tensile strained AlxGa1-xN. The polarization direction of observable surface emission was predicted to alter from c-axis normal (X1) to c-axis parallel (X2) for the middle energy transition (E2) and X2 to X1 for the highest energy transition (E3) between x=0.25 and 0.32. The experimental results were consistently reproduced by the calculation. [ABSTRACT FROM AUTHOR]

Vacancy-type defects in AlN grown by metal-organic vapor phase epitaxy (MOVPE) and lateral epitaxial overgrowth (LEO) using halide vapor phase epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured and compared to the spectra calculated using the projector augmented-wave method. For MOVPE-AlN, the concentration of vacancy-type defects was high near the interface between AlN and the GaN buffer layer, and the defect-rich region expanded from the interface toward the surface when the NH3 flow rate increased. For the sample grown on the AlN buffer layer, however, the introduction of such defects was suppressed. For LEO-AlN, distinct deep emission peaks at 3–6 eV were observed in cathodoluminescence spectra. From a comparison between Doppler broadening spectra measured for LEO-AlN and computer simulated ones, an origin of the peaks was identified as complexes of Al vacancy (VAl) and oxygen atoms substituting nitrogen sites such as VAl(ON)n (n=3 and 4). [ABSTRACT FROM AUTHOR]